Highly loaded fluorescent lamp

ABSTRACT

A highly loaded fluorescent lamp ( 10 ) (FIG.  1 ) comprises a hollow, translucent glass body ( 12 ) containing a medium capable of generating at least several wavelengths of UV radiation. A plurality of phosphors is disposed on the inside surface of the glass body ( 12 ), the plurality of phosphors visible radiation upon exposure to the UV radiation. At least one of the plurality of phosphors is subject to degradation upon lone-term exposure to one of the at least one of several wavelengths of UV radiation. The at least one of the plurality of phosphors subject to degradation is installed (FIG.  12 ) adjacent the inside surface ( 14 ) of the glass body ( 12 ) to form a first layer ( 16 ); and the remainder of the plurality of phosphors is disposed on the first layer to form a second layer ( 18 ), the second layer not being subject to long-term degradation upon exposure to the UV radiation.

This application claims priority from Provisional Patent Application No.60/224,475, filed Aug. 10, 2000.

TECHNICAL FIELD

This invention relates to fluorescent lamps and more particularly tohighly loaded electrodeless fluorescent lamps. By highly loaded is meantlamps having a discharge current of equal to or greater than 2 amperes.

BACKGROUND ART

Electrodeless lamps, such as those shown in U.S. Pat. No. 5,834,905,employ a hollow glass envelope containing mercury vapor and a buffer gasand having a phosphor coating on the inside surface of the glass body.The phosphor is a substantially homogeneous mixture of usually threematerials having emission spectra in different parts of the visiblespectrum and blended to emit white light. In preferred forms of thislamp the blend includes three phosphors, namely, red emitting Y₂O₃:Eu³⁺,green emitting LaPO₄:Ce³⁺,Tb³⁺ and blue emitting BaMgAl₁₀O₁₇:Eu²⁺. Theselamps are designed for extremely long life, i.e., in the neighborhood of100,000 hours. The long life of these lamps has given rise to problemsinvolving a color shift in the emitted spectra. It has been determinedthat this color shift is caused by degradation of some of the lampphosphors during life, the degradation probably being caused by the longexposure to at least one of several wavelengths of ultraviolet radiationgenerated during operation of the lamp.

DISCLOSURE OF INVENTION

It is, therefore, an object of the invention to obviate thedisadvantages of the prior art and to improve the performance andconsistent color output of such lamps.

These objects are accomplished, in one aspect of the invention, by theprovision of a highly loaded fluorescent lamp comprising a hollow,translucent glass body containing a medium capable of generating atleast several wavelengths of UV radiation. A plurality of phosphors isdisposed on the inside surface of the glass body, the plurality ofphosphors emitting visible radiation upon exposure to the UV radiation.At least one of the plurality of phosphors is subject to degradationupon long-term exposure to one of the at least one of severalwavelengths of UV radiation. The at least one of the plurality ofphosphors subject to degradation is installed adjacent the insidesurface of the glass body to form a first layer; and, the remainder ofthe plurality of phosphors are disposed on the first layer to form asecond layer, the second layer not being subject to long-termdegradation upon exposure to the UV radiation.

The deposition of the more stable phosphor on top of the least stablephosphor provides adequate protection of the least stable phosphorduring the life of the lamp, allowing the lamp to more nearly provideits designed color output during its long life.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic plan view of an electrodeless fluorescent lampemploying the invention; and

FIG. 2 is a cross-section taken along the line 2—2 of FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in conjunction withthe above-described drawings.

Referring now to the drawings with greater particularity, there is shownin FIG. 1 a highly loaded electrodeless fluorescent lamp 10 with itsexternal power sources omitted. The lamp 10 has a hollow glass body 12which contains a medium capable of generating at least severalwavelengths of ultraviolet radiation. The medium can comprise mercuryvapor and a buffer gas, usually a noble gas such as krypton; however,argon or other equivalent gases can be used.

Additionally, the inside surface 14 of body 12 of prior art lamps hasbeen coated with a substantially homogeneous blend of three phosphorswhose blended red, green and blue emission provides a balanced whitelight output. Emission from the phosphors is stimulated by exposure tothe ultraviolet radiation, particularly at 185 nm and 254 nm, which isprovided during lamp operation by the mercury vapor. In a preferredembodiment of this invention, the red emitting phosphor is Y₂O₃:Eu³⁺(hereinafter, YOE) the green emitting phosphor is LaPO₄:Ce³⁺,Tb³⁺(hereinafter, LAP) and the blue emitting phosphor is BaMgAl₁₀O₁₇:Eu²⁺(hereinafter, BAM).

As above noted, it has been discovered that during the long lifetime ofthese lamps, an undesirable color shift can occur. It is believed thatthis color shift is caused by a degradation of the blue emittingphosphor due to long exposure to particularly the 185 nm radiation. Ionbombardment is also suspected of being another cause. Additionally,oxidation of the Eu²⁺ could be occurring during lamp processing.

The color shift problem of these highly loaded lamps can be eliminatedor substantially reduced by applying the phosphors in layers with theblue BAM phosphor being applied first and a blend of the LAP and YOEphosphors being applied over it. This is particularly applicable sincethe YOE is a very stable material and attenuates the UV radiation thusreaching the BAM.

This embodiment is shown in FIG. 2 wherein the inside surface 14 of body12 has applied thereto a first layer 16 of a BAM phosphor and hasapplied over it a second layer 18 of a blend of LAP and YOE phosphors.

Alternatively, the BAM and LAP phosphors can be blended and appliedtogether to form the first coat and can be overcoated with the YOE. Theband gap of LAP is greater than 170 nm and that of the YOE is ˜210 nm.Thus absorption of the 185 nm radiation by the YOE is most likelyaccomplished by excitation of electrons in the host lattice. In the caseof the LAP, it is most likely accomplished by impurity excitation. Sincethe absorption strength of the host excitation is one or two orders ofmagnitude higher than that of the impurity related absorption, the YOEwould be a better absorber of 185 nm radiation than LAP, indicating thatthis latter approach may be the most efficient.

While there have been shown and described what are at present consideredto be the preferred embodiments of the invention, it will be apparent tothose skilled in the art that various changes and modification can bemade herein without departing from the scope of the invention as definedby the appended claims.

What is claimed is:
 1. A highly loaded fluorescent lamp comprising: ahollow, translucent glass body containing a medium capable of generatingat least several wavelengths of UV radiation; a plurality of phosphorsdisposed on the inside surface of said glass body, said phosphorsemitting visible radiation upon exposure to said UV radiation, at leastone of said plurality of phosphors being subject to degradation uponlong-term exposure to one of said at least one of several wavelengths ofUV radiation; said at least one of said plurality of phosphors subjectto degradation being installed adjacent said inside surface of saidglass body to form a first layer; and the remainder of said plurality ofphosphors being disposed on said first layer to form a second layer,said second layer not being subject to long-term degradation uponexposure to said UV radiation.
 2. The lamp of claim 1 wherein saidfluorescent lamp is an electrodeless lamp.
 3. The lamp of claim 2wherein said plurality of phosphors comprises individual phosphorsemitting in the red, green and blue portions of the visible spectrum. 4.The lamp of claim 3 wherein said blue emitting phosphor isBaMgAl₁₀O₁₇:Eu²⁺.
 5. The lamp of claim 4 wherein said BaMgAl₁₀O₁₇:EU²⁺is the phosphor subject to degradation.
 6. The lamp of claim 5 whereinsaid remainder of said phosphors comprise Y₂O₃:Eu³⁺ and LaPO₄:Ce³⁺,Tb³⁺.